Arthur (right) and William Lie Zeckendorf have been planning 18 Gramercy Park since 2007 and closed on the site in 2010. About half of the 16 apartments, which start at $9.53 million, are already in contract. Closings should start in the first quarter of 2013.

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After any big, show-stopping number, a great entertainer is left with an impossible question:

What do you do for an encore?

Since creating the most successful residential building in the city (15 Central Park West, which racked up billions in sales and sold out in 2007, with available resales now ranging from $8.5 million for a two-bedroom up to $95 million for a four-bedroom combo), real estate watchers have been anxiously awaiting what comes next from developers Arthur and William Lie Zeckendorf.

They now have a new number: 18 Gramercy Park.

“We wanted to continue what we did at 515 Park [Ave.] and 15 Central Park West — and bring that concept downtown, with a new condo in a prewar building,” Arthur says.

Of course, 18 Gramercy is a different beast than 15 CPW, which was ground-up construction and consisted of more than 200 units, and it’s different from the Zeckendorf brothers’ 515 Park Ave., their 43-story, 36-unit 1998 condo building in Lenox Hill (where an $8.995 million three-bedroom and a $15.5 million five-bedroom are currently on the market). For one thing, it’s smaller — there will only be 16 units: 14 full-floor, 4,207-square-foot, four-bedroom, 5 1/2-bathroom residences, plus a ground-floor maisonette and a duplex penthouse with a 2,000-square-foot terrace and an infinity pool.

But with its Georgian Revival exterior and its location just across the street from Gramercy Park, one can definitely see why the Zeckendorfs were drawn to this particular property. They no doubt fell for the grandeur of its architecture and the accompanying history.

“This neighborhood was established in 1831 as the premier residential neighborhood,” Arthur says.

“The oldest co-op in the city is right there,” adds William, gesturing toward 32 Gramercy Park.

And within moments, the Zeckendorfs are telling the story of how Peter Cooper’s heirs sold the farmland that eventually became Gramercy Park to Samuel Ruggles.

William says 18 Gramercy’s “original purpose was it was built as a women’s hotel. The Salvation Army took it over from [the 1970s] to 2007.”

The building was erected in 1927 and comes with a handsome brick exterior, which had to be painstakingly reproduced. “The brick was fired with coal back in the 1910s,” Arthur says. “It’s why it has this texture.”

“They did repairs [on the exterior] in the ’80s and ’90s,” William says. “They did not work out.”

Reconfiguring the property meant taking the existing 200 or so single rooms and combining them into the floor-through units.

“This apartment was 26 rooms,” says William, sitting on one of the couches in the living room of 18 Gramercy’s second-floor model unit. “I think it was 26 rooms — or 22 — but they were all these tiny rooms with one bathroom.”

A sweeping hallway was run through the length of the apartments, connecting the suites of bedrooms at one end of the unit along Irving Place, with the dining room, kitchen and living room at the other end, perched above Gramercy Park.

The details were all carefully chosen, from the white oak floors with a prewar-type finish, to the marble island and Smallbone cabinets in the kitchen with built-in Wolf ovens and microwaves and Sub-Zero refrigerators. “The look is both modern and classic,” Arthur says.

About half of the units went into contract quickly, with prices starting at $9.53 million for the maisonette and $14.7 million for the full floors. The Post previously reported that the $42 million penthouse is in contract with Houston Rockets owner Leslie Alexander.

“We hope to have closings in January, February or March — at least the first quarter of 2013,” Arthur says. “Two-thirds of the apartments should be finished [by then].”

Of course, the Zeckendorfs haven’t stopped to take a breath — yesterday they unveiled the encore of their encore: an 87-unit, 44-story condo building at 50 United Nations Plaza.

This new project, which will be London-based architecture firm Foster + Partners’ first residential tower in the US, is slated for completion in 2014. It should have units ranging from 1,100-square-foot one-bedrooms to 6,000-square-foot full-floor homes. There will also be a duplex penthouse with close to 10,000 square feet of space.

The 50 UN Plaza building will also include its own garden and 87-car garage.

The Zeckendorfs didn’t need to build a garden for 18 Gramercy — Sam Ruggles already took care of that for them 180 years ago.